With rapid development of radio and network communications technologies, working frequencies of communications products are increasingly high. The increase of the working frequencies of communications products imposes higher requirements on loss control in a transmission process. Back-drilling a PCB is a technological processing manner that effectively reduces the loss of a hole. After the back-drilling, if the length of a redundant hole copper (Stub) above a signal layer is smaller, the loss is less.
In the prior art, a plated through hole (Plated Through Hole, PTH) is back-drilled generally according to pre-computed theoretic back-drilling depth, and the Stub length of the back-drilling is tested according to a conventional back-drilling test coupon (Coupon) on an edge of the PCB production board, and then slicing analysis is performed to rectify the back-drilling depth. Because the workload of depth detection is large, the Coupon back-drilling depth is usually detected through slicing according to production batches. That is, back-drilling depth is not rectified for all PCB boards. Due to unevenness of thickness of PCB boards, the PCB boards in a same batch have different thicknesses to some extent, and the thickness even varies with positions on a same PCB board. The defects of the prior art are: The back-drilling depth acquired according to a test result in a conventional back-drilling Coupon position on an edge of the PCB board deviates from the depth in an actual back-drilling position on the PCB board. For a thicker PCB board, the absolute value of the deviation is greater, and the deviation of the back-drilling depth is greater.
Functions implemented by a PCB board are more and more complex, and the level of integration is increasing. Correspondingly, the PCB board needs to have more layers and be thicker to meet functionality design requirements of the PCB board. However, the deviation of the back-drilling depth acquired according to the test result in a back-drilling Coupon position on an edge of the PCB board is relatively great, and therefore Stub length is relatively great, which causes a significant impact on loss of a hole during high-frequency signal transmission.